This invention relates to force actuators and is particularly, although not exclusively, applicable to linear force actuators suitable for supporting a load carried by a linearly extendible member such as a column or arm.
In the fields of cinematography and TV it is common to provide a mobile pedestal for a camera having a vertically extendible column having two or more interlinked stages and incorporating a gas strut to counterbalance at least partially the load of the camera on the column throughout the range of travel of the camera. In some arrangements a clamp is provided to lock the column in any required position of vertical adjustment to prevent drifting up or down when the camera is not supported by the operator. An additional operation in securing the camera position is however undesirable and there has been a demand for a column in which the camera can easily be set to any position of adjustment throughout its travel and will remain in that position untended and without the need for a locking device. It will be appreciated that as a gas strut extends, the volume of pressurised gas contained within the strut increases and therefore the pressure declines thereby reducing the force available from the strut. To minimize the loss of pressure in the strut, an arrangement has been devised in which the strut is connected to a separate pressurised gas reservoir sufficiently large to ensure that the change in volume of the strut with extension is relatively small in relation to the total volume of the strut and reservoir taken together so that diminution in gas pressure is relatively small and therefore the fall-off in applied force is relatively small. This arrangement has the disadvantage that it is necessary to incorporate a comparatively bulky gas reservoir in the pedestal. In the demand for more and more compact arrangements this has become a serious disadvantage.
In a further construction, the ram of the strut is tapered throughout its length and operates through a seal which can expand and contract radially so that the affected area of the ram increases and decreases with extension and contraction. As a result, the area on which the gas pressure of the ram operates increases and decreases and so the force generated by the ram increases and decreases. It is difficult however to make a seal to cope with a tapered ram reliable particularly over a long period of use. Also whilst such an arrangement may be satisfactory for a two stage column, the problem of providing an effective seal which will cater for the degree of taper required for a three or four stage column poses severe design and material problems for the seal. It has therefore become desirable to find an alternative solution to the problem.